JPH01312100A - Method for descaling stainless steel - Google Patents

Abstract

PURPOSE:To remove oxide scales in a short time and to improve the resistance to initial rusting by applying electrolysis with the use of a processing soln. added with MoClx under specified conditions at the time of removing oxide scales on the surface of stainless steel. CONSTITUTION:A stainless steel cold-rolled, then annealed, and having oxide scales on the surface is passed through the processing soln. obtained by adding MoClx (x is 3-5) to an aq. soln. of an acid such as nitric acid, a nitric and hydrochloric acid mixture, sulfuric acid, and phosphoric acid or the neutral-salt electrolyte such as Na2SO4 to control the C concn. to 0.1-3.6wt.%, a current is applied to the stainless steel alternately used as the anode and cathode at least once at intervals of 5 sec, for example, and an electrolytic current is applied so that the final polarity on the electrolytic cell outlet side is made negative. The oxide scales are removed when the stainless steel is used as the anode, and the resistance to initial rusting is improved by the surface treatment with Mo when the stainless steel is used as the cathode.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for removing scale from stainless steel, and more specifically, for removing surface oxidation scale from a cold-rolled and annealed stainless steel plate, and at the same time improving the corrosion resistance of the stainless steel. It relates to a highly effective and completely novel method for improving.

[Prior Art] Conventionally, methods for removing surface oxidation scale after cold rolling annealing of stainless steel include immersion treatment in alkali molten salt (so-called salt), electrolytic treatment in neutral salt (Na2SO4, etc.) electrolyte aqueous solution (so-called Lusner). Or sulfuric acid, hydrochloric acid,
Generally, immersion in an acid such as nitric acid, phosphoric acid, nitric-fluoric acid, or electrolytic treatment was used in an appropriate combination depending on the steel type.

Recently, in order to improve productivity due to increased speed of production lines, chloride ion sources have been added in the form of chloride (including hydrochloric acid) to these processing solutions, and the remarkable scale erosion function of chlorine ions has been actively utilized. The use of short-time descaling methods is gaining momentum. For example, a method of immersion in a treatment solution consisting of nitric acid-hydrochloric acid monochloride (ferric
8-42777, Japanese Patent Publication No. 6O-54391), and even neutral electrolyte aqueous solution - NaC! Alternatively, a method of electrolyzing in a treatment solution such as KCρ (Japanese Unexamined Patent Publication No. 53-23245) has been disclosed.

However, these conventional techniques have the following problems. That is, (1) First, in the conventional technology, the pickling line running speed is from several tens of meters to 60 to 70 m/min at most, and a long pickling tank is required to achieve higher line roughness. There was a need. Such a function has not been able to be put to practical use in high-speed, high-efficiency production equipment, such as continuous annealing-pickling equipment, which operates at speeds exceeding 100 m/min.

Furthermore, (2) These conventional techniques cannot meet the requirements of improved initial rust resistance, which has been recently demanded by customers for seat heights.

[Problems to be Solved by the Invention] By the way, as a solution to the two problems of corrosion resistance (2), there is a method of performing surface treatment using molybdenum as a separate process after scale removal treatment. For example, cathodic treatment of stainless steel in phosphoric acid containing molybdate (
JP-A-57-26196) or a method of cathodic treatment of stainless steel using metallic molybdenum as an anode in phosphoric acid (
JP-A-57-41398) and the like are disclosed. All of these methods improve corrosion resistance by forming a molybdenum-containing film on the surface of stainless steel, but the following problems remain:1. The law is applicable to stainless steel that has already been descaled, and the treatment requires a dedicated tank separate from the descaling tank, requiring new equipment.
must be established. In addition, because this treatment cannot be carried out simultaneously with scale removal in one step, the number of steps in the production process increases, and the slow processing speed reduces productivity, causing a significant increase in production cost by 1. .

[Problem to be Solved by the Invention 1] As mentioned above, in systems containing chlorides in the processing liquid,
With the conventional technology related to the ``scale removal of stainless steel'' method, the major problem remained that it was not possible to improve the corrosion resistance of the surface of the target material at the same time without impairing productivity. .

The present invention was developed in view of the current situation, and solves the problems remaining in the known prior art, and removes surface oxidation scale from stainless steel in an extremely short time. The object of the present invention is to provide a rapid and innovative scale removal method that can also improve corrosion resistance.

[Means for Solving the Problems] The present inventors conducted intensive and detailed experiments and studies on an electrolytic scale removal treatment method in a treatment solution containing a chlorine source and a surface treatment method for improving corrosion resistance using molybdenum. As a result of research, we discovered that by electrolytically descaling stainless steel using a treatment solution to which molybdenum chloride was added, scale removal could be completed in a short period of time, and the problem of improving corrosion resistance could also be solved.

That is, to be more specific, the present invention includes: (1) using a solution prepared by adding molybdenum chloride to an acid such as nitric acid, nitric hydrochloric acid, sulfuric acid, or phosphoric acid or a neutral salt electrolyte solution such as sodium sulfate or sodium nitrate; ■ Stainless steel has positive and negative polarity in such a treatment solution.
This is a method for removing scale from stainless steel, which is characterized by electrolytic treatment so that the final polarity at the exit side of the electrolytic cell is negative.

[Function] The function of the present invention will be specifically explained below. In the present invention, molybdenum chloride is used as the chloride or chlorine source. Molybdenum chloride is suitable as a compound that contains chlorine, which is necessary for scale removal, and also contains molybdenum, which is necessary for surface treatment to improve corrosion resistance.

The composition of molybdenum chloride is M o CQ 3, M
o Cg 4, Mo (125, etc.) Any structure may be used as it does not affect the achievement of the purpose of the present invention, but each structure has a difference in solubility in aqueous solution, acid, etc., so it is necessary to remove scale. Depending on the treatment liquid, the amount of G to be used may be adjusted by taking into consideration the concentration of molybdenum chloride added and cost.

The concentration of molybdenum chloride added is determined from two points: securing the chlorine concentration necessary for scale removal and securing the molybdenum concentration necessary for surface treatment to improve corrosion resistance.

First, the chlorine concentration for scale removal is nitric acid, nitric acid,
It changes little by little depending on the treatment solution made of an acid such as sulfuric acid or phosphoric acid or a neutral salt electrolyte aqueous solution.

That is, when a nitric acid aqueous solution is used as the treatment liquid, the relationship between the nitric acid concentration and the appropriate chlorine concentration is as shown in FIG.

When a nitric acid aqueous solution is used as the treatment liquid, the relationship shown in FIG. 1 can be applied as is.

When a sulfuric acid aqueous solution is used as the treatment liquid, the relationship between the sulfuric acid concentration and the appropriate chlorine concentration is as shown in FIG.

Further, when a phosphoric acid aqueous solution is used as the processing liquid, the relationship shown in FIG. 3 is obtained.

Further, when a neutral salt electrolyte (sodium sulfate) aqueous solution is used as the treatment liquid, the relationship between the neutral salt electrolyte concentration and the appropriate chlorine concentration is shown in FIG.

As described above, the appropriate chlorine concentration for obtaining good scale removal properties differs depending on the treatment solution, but the chlorine concentration range for these acid or neutral salt electrolytes is 0.1 to 3. .6% by weight is desirable. If it is less than 0.1% by weight, scale remains, and if it exceeds 3.6% by weight, the surface condition of the stainless steel will deteriorate, such as an over-pickled skin.

On the other hand, the necessary molybdenum concentration for surface treatment to improve corrosion resistance varies depending on the acid or neutral salt electrolyte aqueous solution treatment solution, but for example, when using a nitric acid aqueous solution as the treatment solution, the above scale removal is possible Based on chlorine concentration range <M.

Regarding Cf25 addition concentration, as shown in Table 1, it is clear that the higher the concentration, the greater the improvement in the corrosion resistance of stainless steel, but even the lowest addition amount of M o Cβ5 is more effective than descaling stainless steel using hydrochloric acid addition system. It can be seen that the corrosion resistance is also significantly improved.

On the other hand, apart from scale removal properties, Mo
Looking at the results of investigating the relationship between weight and corrosion resistance (Table 2), we find that
As shown in Table 1, it can be seen that the corrosion resistance improves as the Mo concentration increases, but if the amount of Mo added is less than 2 g/ff, the corrosion resistance improves significantly, but it is not sufficient. It is believed that adding more than 10 g/8 will give a completely sufficient improvement in corrosion resistance and the result will be saturated.

Exactly the same results were obtained with other acid or neutral salt electrolytes as to the relationship between the amount of Mo and the improvement in corrosion resistance.

Considering these study results, it is not necessary to specify the amount of Mo added to the treatment liquid in the form of molybdenum chloride in the present invention, since the effect is recognized even in a small amount, but it is preferably 2 g/j2. By setting Log/ to 9 or less, there are great advantages in terms of improved corrosion resistance and economical efficiency.

The properties required of the treatment liquid are basically that it is a system that can electrolytically remove scale from stainless steel, and it can be used with conventionally used acid aqueous solutions such as nitric acid, nitric-hydrochloric acid, sulfuric acid, and phosphoric acid, or with N a 2 A neutral salt aqueous electrolyte solution such as S 2 O 4 can be used.

The concentration of these acid or neutral salt electrolyte aqueous solutions is not particularly defined in the present invention, and may be within the range of 5 to 20% by weight, which is generally used as an appropriate concentration range. The inventor's studies have revealed that there is no problem with the effectiveness of the present invention within a general concentration range, as shown above.

The processing solution used is acidic or neutral. Figure 5 shows the pH
The test conditions are as follows.

Test material: 5US304 steel, annealed steel strip treatment liquid composition concentration Na2504: 200g/ff MoCff5: 15g/[ Electrolysis conditions Liquid temperature: 80°C Electrolysis pattern: Cathode 5 seconds - Anode 5 seconds - Cathode 5 seconds Current density: I 5A/drn' The scale removability is expressed by evaluation symbols as follows.

○ No scale remaining △ Small scale remaining × Large scale remaining
2371 method), evaluation was made based on the area ratio of rust occurrence after 96 hours, and the evaluation symbols were as follows.

C0% ○ 1-4% △ 5-9% As a result of carrying out the present invention, scale removal is completed in acidic and neutral regions, and corrosion resistance is also improved.
When H is in the alkaline region of 9 to 10 or more, the scale removability decreases, the occurrence of scale remains increases, and the effect of improving corrosion resistance is low ((It was found that there is a ψ direction). From this result, it is necessary to use acidic or neutral salt electrolyte water (8 liquids) as a suitable treatment liquid in order to fully achieve the purpose of the present invention.

The electrolysis method of the present invention must be such that the stainless steel to be scaled has both positive and negative polarity, and the final polarity at the exit side of the electrolytic cell is cathode. In other words, in order to dissolve and remove the surface oxide scale of stainless steel by electrolysis, it is necessary to use the stainless steel as an anode, and in order to achieve stainless steel surface treatment with molybdenum, the stainless steel must be used as a cathode. It is necessary to do so. Furthermore, in order to utilize the corrosion-resistant layer of molybdenum on the surface of stainless steel without degrading it and to make use of it in the properties of finished products, the final polarity during electrolysis must be cathode. If the final polarity becomes even slightly positive, the corrosion-resistant layer of molybdenum formed from an extremely thin film will be dissolved. For this reason, the electrolysis method of the present invention was set as described above.

Note that although the treatment time for each anode and cathode polarity or the polarity pattern excluding the final polarity may cause a difference in the degree of achievement in achieving the purpose of the present invention, the basic effect is achieved. Although there is no particular limitation in the present invention since there is no difference in terms of It is good to do so.

By the way, it is not clear at present how molybdenum acts on the stainless steel surface and improves its corrosion resistance through cathodic electrolysis, but IMMA (Jon) M
Ass Micro Analysis) The results (Figure 6) show that molybdenum (Mo) is present on the stainless steel surface layer.
The surface molybdenum layer is believed to contribute to improving corrosion resistance. The test conditions for Section 6 are as follows.

Test material SUI (409 steel Annealed steel strip treatment solution composition concentration Sulfuric acid: 50g/2 MoCI25: 5g/12 Electrolysis conditions Solution temperature: 50°C Electrolysis pattern.

Anode 5 seconds - Cathode 5 seconds Current density 15A/drn2 It is generally said that molybdenum does not deposit alone (as a single metal) in an aqueous solution, but takes the form of induced eutectoid deposition with iron group metals. (References, for example, 77th Metal Surface Treatment Technology Association Lecture Conference Manuscript Collection P8). The molybdenum precipitated in the present invention is also considered to have such a form, and such a precipitated layer is considered to constitute a surface layer exhibiting corrosion resistance.

Regarding the temperature of the treatment solution in electrolytic treatment, a higher temperature tends to be more favorable for the effects of the present invention, but the temperature of the solution normally used for removing scale from stainless steel, that is, 40 to 80°C Sufficient effects are recognized and there are no operational problems (this is preferable).

In addition, the larger the current density during electrolysis, the better it is for the effects of the present invention to appear, but if it is too large, problems such as rough skin, exhaust gas treatment, and cost problems will occur. As used, 5-30A
/drn” is preferable.

The present invention, which is characterized by adopting the form described above, can be applied to commonly used alkaline molten salt immersion treatment, electrolytic treatment in a neutral salt electrolyte aqueous solution, sulfuric acid immersion, electrolytic treatment, etc. When combined with pre-treatment, the effect is further enhanced.

[Example] Examples of the present invention will be described below.

Example-1 5US continuously annealed under normal annealing conditions after cold rolling
After electrolytically treating a 430 steel strip using a nitric acid-chloride based acid as the treatment liquid in Examples and Comparative Examples, a salt spray test (LT T 522371) was conducted to determine the scale removal performance and corrosion resistance. compared. The results are shown in Table 3.

As a result, it was confirmed that the electrolytic treatment using a nitric acid-molybdenum chloride system according to the present invention not only completes scale removal in a short period of time, but also achieves an improvement in corrosion resistance that could not be achieved with conventional techniques.

Example 2 A steel strip of 5US304 steel that was batch annealed under the same normal annealing conditions as in Example 1 was subjected to electrolytic treatment using an example and a comparative example in which the treatment liquid was a sulfuric acid-chloride acid. After that, a salt spray test was conducted to compare scale removal performance and corrosion resistance. The results are shown in Table 4.

As a result, it was confirmed that the electrolytic treatment using the sulfuric acid-molybdenum chloride system according to the present invention not only completes scale removal in a short time, but also achieves an improvement in corrosion resistance that could not be obtained with the conventional techniques.

Example-3 An example and a comparative example in which the treatment liquid was a nitric acid-hydrochloric acid monochloride-based acid were carried out on a 5UH409 steel strip that was continuously annealed under the same normal annealing conditions as in Example-1. After electrolytic treatment using the same material, a salt spray test was conducted to compare scale removal performance and corrosion resistance. The results are shown in Table 5.

As a result, it was confirmed that the electrolytic treatment using the nitric acid-hydrochloric acid-molybdenum chloride system of the present invention not only completes scale removal in a short time, but also achieves improvements in corrosion resistance that could not be obtained with conventional techniques. .

Example-4 The same 5UH409 steel strip used in Example-3 was subjected to electrolytic treatment using an example and a comparative example in which the treatment liquid was a phosphoric acid-chloride acid, and then a salt spray test was conducted. carried out,
The scale removability and corrosion resistance were compared. The results are shown in Table 6.

As a result, it was confirmed that the electrolytic treatment using a phosphoric acid-chloride system according to the present invention not only completes scale removal in a short time, but also achieves an improvement in corrosion resistance that could not be obtained with the prior art.

Example-5 The same 5US430 steel strip used in Example-1 was electrolytically treated using an example and a comparative example in which the treatment liquid was a Na2504-chloride-based aqueous solution, and then a salt spray test was conducted. The scale removability and corrosion resistance were compared. The results are shown in Table 7.

As a result, it was confirmed that the electrolytic treatment using the Na2504-molybdenum chloride system according to the present invention not only completes scale removal in a short time, but also achieves an improvement in corrosion resistance that could not be obtained with the conventional technique.

[Effects of the Invention] As described above based on the examples, the present invention provides a method for electrolytically removing scale from cold-rolled and annealed stainless steel strip in a treatment solution containing chlorides. ! This proposes a new UI-like method, which can achieve the following effects. That is, (one) by the method of the present invention,
In addition, it is possible to remove kale and improve corrosion resistance (improvement) in the same process.

(It is a simple two-liquid, one-step processing method that does not require any new equipment investment, and can significantly increase productivity and reduce production costs.

(J　Electrolytic treatment time is short and high-speed threading is possible.

It has the following effects ('), and its contribution to industry is extremely large.

It goes without saying that the addition of molybdenum chloride according to the present invention can also be applied to the conventional technique of mixing and adding chlorides such as Fe(, j23, NaCff, etc.) into a processing solution.

[Brief explanation of the drawing]

Figure 1 shows the case when nitric acid is used as the treatment liquid, Figure 2 shows the case when sulfuric acid is used as the treatment liquid, Figure 3 shows the case when phosphoric acid is used as the treatment liquid, and Figure 4 shows the case when the treatment liquid is neutral salt electrolyte (sodium sulfate). A graph showing the relationship between each scale removal property and the acid or neutral salt electrolyte concentration - chlorine concentration when used as a treatment liquid.
Fig. 6 shows the surface IM of stainless steel treated according to the present invention.
It is a graph showing the analysis results in the MΔ depth direction.

Claims (1)

[Claims] 1. When removing scale from stainless steel in a treatment solution consisting of an acid or neutral salt electrolyte aqueous solution, molybdenum chloride is added to the treatment solution, and the stainless steel is made to have positive and negative polarity at least once, A method for removing scale from stainless steel, characterized by performing electrolytic treatment with the final polarity at the exit side of the electrolytic cell being negative.